1. Field of the Invention
This invention relates generally to the field of control electronics for an induction motor, and particularly to a circuit for controlling the slip frequency of the motor to eliminate clipping of the AC motor drive signals throughout the operating range of the motor.
2. Background Art
Induction motors commonly employ an electronic power inverter to convert DC voltage from a power source, such as a battery, to AC voltage/current for energizing the motor windings. Motors of this type may be used in either fixed speed or variable speed drives. Applications for such drives are widespread and include electric vehicles and industrial process drives.
FIG. 1 illustrates a typical motor control circuit for a three-phase induction motor 10. A three-phase power inverter, shown generally as 12, comprises an array of solid state switches S1-S6, which are typically power transistors, arranged in a bridge configuration. Energy is supplied to the power inverter from a voltage source, such as battery 14. Control electronics 16 provide a control signal to each of the inverter switches to modulate the drive signals applied to the motor windings. Typically, the drive signals are pulse width modulated at a switching rate on the order of 20 kHz to produce sinusoidal current waveforms across the motor windings with amplitude and frequency so as to produce the desired speed and torque from the motor. Inputs to control electronics 16 include the sensed rotor speed of the motor and a slip frequency, which is the difference between the rotor frequency and the frequency of the rotating magnetic field generated by the currents in the stator windings.
Regulation of the motor drive signals is achieved by varying the duty cycle of the switch pairs S1/S2, S3/S4 and S5/S6. The peak amplitude of the output voltage is limited by the DC input voltage supplied by battery 14. If the commanded speed and torque are such that the motor controller seeks to develop an output voltage greater than can be supplied by maximum modulation by the drive signals (i.e., 100% duty cycle), the voltage waveforms of the motor drive signals are distorted from the optimum sinusoidal waveforms with the peak voltage "clipped" at the DC input voltage level. Clipping of the output voltage causes reduced motor efficiency, increased torque pulsation and potential instabilities. These detrimental effects are caused by the introduction of high frequency components in the drive signal waveform.
In order to avoid such effects, some prior art motor drive systems implement a slip frequency control algorithm to prevent clipping. Such an algorithm seeks to decrease the effective impedance of the motor by adjusting the slip frequency at high commanded currents so that the peak voltage is held below the DC input voltage. An increase in the slip frequency will decrease the effective impedance of the motor. However, since the impedance of the motor varies not only with slip frequency, but also with the motor speed, current level and temperature, a typical prior art anti-clipping algorithm, which is intended to avoid clipping under extreme conditions, will have a substantial margin of safety throughout much of the operating range of the motor. This margin results in less than maximum modulation in certain cases where full power is desired. Thus, the maximum power output of the motor may be needlessly limited in portions of its operating range.